Unveiling the prevalence of larger magnitude earthquakes preceding major shallow seismic events (M≥7.2) in Japan (1995–2021)

We conducted a study using data from the Seismological Bulletin of Japan, which is maintained by the Japan Meteorological Agency. We focused on major earthquakes (with a magnitude of 7.2 or higher) that occurred in or around Japan, specifically within the coordinates of 18–50 °N and 119–158 °E, between 1995 and 2021. For each of these mainshocks, we analyzed seismic activity patterns within a 100 km radius, using two different time periods. To enhance our investigation, we also employed a space–time cluster detection method initially designed for identifying clusters in epidemiology, enabling us to reclassify earthquakes into sequences for further analysis. Unlike conventional fixed-window approaches, this method (SaTScan™) statistically detects spatio-temporal clusters without predefined spatial or temporal boundaries, allowing for a more flexible and data-driven classification. Our main discovery is a significant increase in the magnitude of the largest earthquake and the observed seismic energy during the 3–0 month interval, compared to the values during the 6–3 month interval before the mainshock respectively. However, we did not find any stable or significant differences in the numbers of events, mean magnitude values, median magnitude values, and $b$-values.